The present invention relates to a process for producing filaments employing a modified meltblown process and more particularly to a process for producing lyocell filaments employing a modified meltblown process that mechanically attenuates the filaments.
In the past decade, major cellulose fiber producers have engaged in the development of processes for manufacturing shaped cellulose materials including filament and fibers based on the lyocell process. One process for producing lyocell filaments known as a meltblown process can be generally described as a one step process in which a fluid dope is extruded through a row of orifices to form a plurality of filaments while a stream of air or other gas stretches and attenuates the hot filaments. The latent filaments are treated to precipitate the cellulose. The filaments are collected as continuous filaments or discontinuous filaments. Such a process is described in International Publication No. WO 98/07911 assigned to Weyerhaeuser Company, the assignee of the present application.
Lyocell filaments produced by an existing meltblown process are characterized by variability in diameter along their length, variability in length and diameter from filament to filament, a surface that is not smooth and a naturally imparted crimp. In addition it has been observed that lyocell filaments made by a meltblown process exhibit fibrillation at desirably low levels. These properties of lyocell filaments produced by known meltblown processes make them suitable for applications where such properties are desirable; at the same time these properties make the meltblown lyocell filaments less suitable for other applications where less variability in filament diameter, less natural crimp and higher strength are desired.
Another process for making lyocell filaments is known as dry-jet wet spinning. An example of dry-jet wet processes is described in U.S. Pat. Nos. 4,246,221 and 4,416,698 to McCorsley III. A dry-jet wet process involves the extrusion of a fluid dope through a plurality of orifices to form continuous filaments in an air gap. Usually the air in this gap is stagnant, but sometimes air is circulated in a direction transverse to the direction that the filaments are traveling in order to cool and toughen the filaments. The formed continuous filaments are attenuated in the air gap by a mechanical tensioning device such as a winder. A tensioning device has a surface speed that is greater than the speed at which the dope emerges from the orifices. This speed differential causes the filaments to be mechanically stretched resulting in a reduction in the diameter of the filaments and the strengthening thereof. The filaments are then taken up by a conveyer or other take up device after they have been treated with a non-solvent to precipitate the cellulose and form continuous filaments. These filaments can be gathered into a tow for transport and washing. Staple fibers can be made by cutting a tow of the filaments. Alternatively, the continuous filaments can be twisted to form a filament yarn.
Lyocell filaments formed by a dry-jet wet process are characterized by a smooth surface and little variability in cross-sectional diameter along a filament length. In addition, diameter variability between dry-jet wet filaments is low. Further, lyocell filaments from the dry-jet wet process have little if any crimp, unless the filaments are post-treated to impart such crimp. It is believed that the susceptibility of lyocell filaments made by a dry-jet wet process to fibrillate is greater than the susceptibility of fibers made by known meltblown processes to fibrillate. Therefore, while lyocell filaments made by a dry-jet wet process or lyocell fibers made from such filaments may be preferred for applications where low natural crimp, smooth surfaces, low variability in cross sectional diameter along a fiber and low variability in diameter from fiber to fiber are desirable, they still may be more susceptible to fibrillation compared to lyocell fibers made using known meltblown processes.
As demand for lyocell fibers increases and broadens there is a need for improved methods of producing lyocell fibers that are capable of producing fibers with desirable properties and without those undesirable properties that are imparted to the fibers by existing processes for producing lyocell.
The present invention provides such an improved method of producing lyocell filaments that includes the steps of extruding a dope through a plurality of orifices into a stream of gas to form substantially continuous elongate filaments. The gas stream attenuates and at times stabilizes the extruded filaments. In addition, in accordance with the present invention, the filaments are mechanically attenuated using a winder or other type of take-up device. The mechanical winder or other take-up device applies an external force to the filaments in a direction parallel to the length of the filaments. This force is in addition to the force applied by the gas stream or gravity. Lyocell filaments produced by a process carried out in accordance with the present invention and lyocell fibers cut from such filaments exhibit desirable properties such as low susceptibility to fibrillation, smooth surfaces, low variability in cross-sectional diameter along the filament or fiber length and from fiber to fiber and little natural crimp. In addition, the filaments and fibers possess strength properties that make them suitable for many applications where lyocell filaments and fibers are presently used or contemplated.
A further advantage of the present invention is that it will enable higher speed spinning of lyocell filaments compared to the speed at which filaments are spun using conventional dry-jet wet or melt blowing processes. Higher speed spinning will result in increased production rates by increasing dope throughput. Alternatively, if dope throughput is not increased, fiber diameter can be decreased.
The degree to which the extruded filament is attenuated by the gas and the degree to which the filament is attenuated mechanically in accordance with the present invention can vary. For example, in certain embodiments it may be preferred that the gas provides most of the attenuation with little mechanical attenuation. In other situations it may be preferred that little attenuation results from introducing the extruded filament into the gas stream and that most of the attenuation be provided mechanically.
Bicomponent cellulose filaments comprising cellulose and other polymers and filaments comprising blends of cellulose and other materials can also be produced using a process carried out in accordance with the present invention by forming dopes from combinations of cellulose with other polymers.